EMC Question of the Week: December 22, 2025

In a waveguide made of a single conductor such as a hollow tube or a rectangular duct, electromagnetic energy can only propagate  

1. at one frequency
2. at specific discrete frequencies
3. below a certain cutoff frequency
4. above a certain cutoff frequency

Answer

The best answer is “d.” Waveguides made from a single conductor support different modes of propagation. Each mode has a cutoff frequency. Energy can only propagate in that mode at frequencies above the cutoff frequency.

For a hollow tube with a circular cross section, the mode with the lowest cutoff frequency is the TE₁₁ mode, which has a cutoff frequency equal to

$f_c=1.8412\frac{c}{2\pi r}$

where c is the free space velocity of light and r is the inside radius of the tube.

For a rectangular waveguide, the lowest-order propagation mode is the TE₁₀ mode with a cutoff corresponding to the frequency at which the largest cross-sectional dimension is equal to a half wavelength.

Shielded enclosures sometimes rely on waveguides operating below cutoff to provide ventilation. Apertures with significant depth can effectively block electromagnetic wave propagation at wavelengths larger than the aperture cross-section.

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